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P0842 — Transmission Fluid Pressure Sensor/Switch A Circuit Low

Moderate

Quick answer

P0842 means the signal from transmission fluid pressure sensor/switch A is stuck low — the computer can’t confirm hydraulic pressure in the circuit that switch watches. Check fluid level and condition first, then the switch’s connector and wiring. On many vehicles the fix is an inexpensive screw-in pressure switch, not internal transmission work.

What it means

Many automatics verify their own hydraulics with pressure switches: small sensors on the case or valve body that report when a given clutch circuit actually has pressure. P0842 sets when the signal from switch A reads low when it shouldn’t — either that circuit genuinely has no pressure, or the switch and its wiring are lying about it.

Which circuit “A” watches varies by transmission, so check your service information. A well-known example: on many 2000s Honda and Acura automatics, this pair of codes (P0842/P0847) points at individual clutch pressure switches — inexpensive screw-in parts on the transmission case that fail far more often than the hydraulics they measure.

Because the switch exists to confirm pressure, the computer takes a low reading seriously: many vehicles respond with harsher shifts or limp mode to protect the clutches — even when the only real failure is a $30 switch.

P0842 symptoms: what you'll notice

  • Check engine light — often the only symptom when the switch itself is what failed.
  • Harsh or oddly timed shifts once the computer falls back to protective shift pressure.
  • A flashing gear indicator (the D light) on some vehicles — their way of flagging a transmission fault.
  • Limp mode in stubborn cases: one held gear and raised RPM.

Common causes

Ordered from most to least likely.

  1. 1.

    Low or degraded transmission fluid

    First, as always: genuinely low pressure from low fluid reads “low” at the switch — and it’s the cheapest fix on this list.

  2. 2.

    Failed pressure switch

    The most common confirmed repair. These small case-mounted switches live in heat and constant pressure cycling, and they simply wear out.

  3. 3.

    Wiring or connector damage

    A chafed wire shorted to ground or a corroded pin writes the same low-signal signature as a dead switch.

  4. 4.

    Valve body or clutch circuit genuinely losing pressure

    The serious version — it usually arrives with shift complaints or a ratio code, not just a light.

How to fix it: diagnosis, step by step

Cheapest and most likely checks first.

  1. 1 Check the fluid first

    Level per your vehicle’s procedure, color, and smell. If the fluid is low, fix that and the leak before trusting any electrical theory — the switch may be reporting the truth.

  2. 2 Read the code pattern

    P0842 alone points at one switch or its wiring. P0842 and P0847 together is suspicious — two switches rarely die at once, so look at shared wiring or genuine low pressure. A ratio or slip code alongside says the pressure loss is real.

  3. 3 Inspect the switch connector and wiring

    Find the switch (case-mounted on many vehicles), unplug it, and look for corrosion, fluid intrusion, or a chafed harness nearby. Free, and a frequent ending.

  4. 4 Test the switch and replace if it fails

    With service info, a multimeter confirms whether the switch opens and closes as pressure rises. Case-mounted switches are usually an easy DIY: drain a little fluid, swap the switch, top off.

  5. 5 Pressure-test the circuit if the code survives a new switch

    A new switch plus good wiring plus the code returning means the circuit truly lacks pressure — time for a line pressure test and a transmission specialist.

Parts & tools you may need

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Related codes

Frequently asked questions

What does code P0842 mean?
P0842 means the signal from transmission fluid pressure sensor/switch A is stuck low — the computer can’t confirm hydraulic pressure in the circuit that switch watches. It’s moderately serious — you can usually keep driving gently, but diagnose it soon.
How much does it cost to fix P0842?
The switch itself typically costs $15–60, and case-mounted ones replace in under an hour — many DIYers do it in the driveway with a wrench and a fluid top-off. Shops commonly charge $100–250 for the job. Only the rare genuine-pressure-loss case costs more.
Is my transmission dead?
Almost certainly not. This is a circuit-low code, and the overwhelming majority trace to the switch, its connector, or low fluid — not internal damage. A failing transmission announces itself with slipping and ratio codes, not just a pressure-switch signal.
Can I drive with P0842?
Generally yes, gently and short-term — especially if shifting feels normal. If the transmission goes into limp mode or shifts harshly, treat that as your deadline: the computer is protecting the clutches because it can’t confirm pressure.
Why do I have P0842 and P0847 at the same time?
Two pressure switches rarely fail simultaneously. When both codes appear, suspect what they share: a common harness section or connector, a computer input issue, or genuinely low pressure — which brings you right back to checking the fluid first.
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